The present invention relates to filtering techniques used in data communications.
Over the coming years, more sophisticated and more complex data services will be offered not only to the business sector but to sector residential sector as well. As a result, data communications equipment will be required to support higher and higher data rates in order to bring these sophisticated and complex data services to the home.
Typically in the residential sector, a full-duplex modem is used to deliver data services over a single communications channel, which is typically a twisted pair of telephone wires. In order to accommodate full-duplex operation, signal energies from both directions of transmission must be superimposed on this single communications channel simultaneously. As a result, some signal energy from each direction of transmission inevitably leaks through to the opposite direction path. This cross-path leakage allows some of a modem's transmitted signal to appear at the input of its own receiver. This undesired leaked-through signal is often referred to as "echo." An echo signal can seriously degrade a modem receiver's performance if not mitigated. Although the echo cannot simply be filtered, it can be reduced to acceptable levels through the use of an adaptive filter that is configured as an "echo canceler" within the modem. The echo canceler adaptively synthesizes a replica of the echo, which is then subtracted from the received signal, leaving a substantially echo-free signal. This echo-compensated signal is applied to the modem's data receiver. The echo canceler technique has been so successful that it now is a standard component in most full-duplex high-speed modems.
In addition to the presence of echo, the presence of amplitude and delay distortion in the transmission medium gives rise to intersymbol interference. Intersymbol interference results in errors in determining the value of the transmitted signal at the receiver since adjacent transmitted symbols begin to overlap with one another. To combat the effects of amplitude and delay distortion, another form of an adaptive filter, known as an equalizer, is typically used in a modem.
Today, the adaptive filter, whether for echo canceling or equalization, is typically a digital adaptive filter, which is required to perform a number of filtering operations in each sample interval of an input signal. At the heart of the filtering operations is the completion of numerous multiplication operations within the sampling interval. The speed of a multiplication operation is dependent on the size, i.e., the dynamic range or resolution, of the numbers that are being multiplied together. Consequently, as data rates increase to support new data services, the amount of time, i.e., the sampling interval, available to complete the numerous multiplications decreases. Eventually, the sampling interval becomes so short that the existing digital adaptive filter must be redesigned in order to perform the required filtering operations at the given dynamic range. Unfortunately, any redesign entails additional design and implementation cost. Similarly, as greater accuracy is demanded, a larger dynamic range is required and that also increases the integrated circuit complexity because more calculations are required within the same sampling interval.